Process for producing powdered superalloys

ABSTRACT

A process produces the pre-alloyed superalloy powders having the constituents homogeneously dispersed throughout and of high purity by placing a shaped article of preselected metallic content in an induction furnace, melting the article by induction heating in a relatively short period of time, forming droplets of molten metal, subjecting the molten metal to a centrifugal force by a rotating disk, cooling the droplets into superalloy particles, collecting the metal particles and providing an inert gas having less than 1 ppm of oxygen, water and nitrogen throughout the foregoing steps.

United States Patent Smith PROCESS FOR PRODUCING POWDERED SUPERALLOYS [75] Inventor: James Thomas Smith, Towanda, Pa.

[73] Assignee: GTE Sylvania Incorporated,

Stamford, Conn.

[22] Filed: May 24, 1971 [21] Appl. No.: 146,142

[52] US. Cl. 264/8 [51] Int. Cl B0lj 2/04 [58] Field of Search 264/8 [56] References Cited UNITED STATES PATENTS 2,897,539 8/1959 McMillan 264/8 3,346,673 10/1967 Last et a]. 264/8 3,501,560 3/1970 Howes et al. 264/8 Mar. 18, 1975 Primary Examiner-Robert F. White Assistant ExaminerJ. R. Hall Attorney, Agent, or Firm-Norman .I. OMalley; Donald R. Castle; William H. McNeill [57] ABSTRACT A process produces the pre-alloyed superalloy powders having the constituents homogeneously dispersed throughout and of high purity by placing a shaped article of preselected metallic content in an induction furnace, melting the article by induction heating in a relatively short period of time, forming'droplets of moltenmetal, subjecting the molten metal to a centrifugal force by a rotating disk, cooling the droplets into superalloy particles, collecting the metal particles and providing an inert gas having less than 1 ppm of oxygen, water and nitrogen throughout the foregoing steps.

5 Claims, 1 Drawing Figure SUPERALLOY BAR POWER MAKING SUPERALLOY POWDER PMEMEWR 1 81915 3.872193 SUPERALLOY BAR IO POWDER MAKING SUPERALLOY POWDER INVENTOR. JAMES T.. SMITH M I ,4. BY pm PROCESS FOR PRODUCING POWDERED SUPERALLOYS CROSS-REFERENCES TO RELATED APPLICATIONS this application, disclosed that the improved superal-,

loys having certain nitrides and the process for preparing same are disclosed.

BACKGROUND OF THE INVENTION This invention relates to the production of prealloyed nickel base alloy powders known as superalloys. More particularly, it relates to the production of a superalloy powder having an oxygen content of less than 100 parts per million. i

Superalloys is a generic name given to certain nickel base alloys having a unique microstructure. The alloys are further characterized by their heat resistance and high strength. These alloys generally contain from about 50 to about 70 weight percent of nickel alloyed with varying amounts of chromium, cobalt, aluminum, titanium, molybdenum, tungsten, niobium, tantalum, boron, zirconium and carbon. The metallurgical composition of superalloys are known such as those compiled in the ASTM Subcommittee XII Report, published by the Metal Processing Division of Curtiss Wright Corporation. In the 8th edition of the Metals Handbook superalloys" are defined as alloys developed for high temperature service where relatively high stresses (tensile, thermal, vibratory and shock) are encountered and where oxidation resistance is frequently required, and is the definition used herein. Typical alloys are supplied by a variety of suppliers under tradenames of lN-lOO, Astroloy, etc.

There are several methods for making powders. For example, the particular material such as the alloy is atomized after melting the particular material and forcing these molten metals through an orifice. The interaction ofthe high pressure gas or liquid with molten metal as it passes through the orifice causes the melt to be broken up into powder particles. Another method is the rotating electrode process such as that disclosed in US. Pat. No. 3,099,041. A further method is to take chips of the desired materials and mechanically abrade them into powder. Still an additional method is the pressurization of a molten material by a gas and spraying the metal into a vacuum chamber where powder particles are cooled and collected as disclosed in US. Pat. No. 3,510,546. In US. Pat. No. 3,472,922 a method is disclosed wherein a metal salt is melted, formed into droplets and the droplets are reacted to form a metal or a metal compound powder. The pri mary disadvantage of the foregoing methods is that often the powder does not have the same analysis as the starting material. For example, in the atomization method the molten powder is held in a refractory oxide crucible prior to forming the powder. During the holding time the composition of the alloy can change due to the volatilization of certain alloy ingredients. Additionally, during the atomization there can be considerable oxygen and other gases brought into the powder while the inert gas reacts with the molten material. The rotating electrode process also has its disadvantages of the introduction of oxygen and impurities from the impure inert gas used during the powder making process. The abrading of chips of material generally results in powders which are high in oxygen and, other impurities. In the melting and reaction of salts the problem inherent in this process is also one of impurities which are brought into the process during the reaction or else due to the inability of the reaction process to reduce all of the interstitial gas content to a desirable level.

It is believed, therefore, that a prealloyed metal powder having a low metallic impurity content and an oxygen content of less than parts per million and the process for producing same are advancements in the art.

SUMMARY OF THE INVENTION According to one aspect of this invention there is provided a superalloy powder having an oxygen imput rs ntent he 9 Hal??? mi l n.

According to another aspect of this invention, there is provided a process for producing the powder, said process comprising placing a shaped article of a high purity, superalloy having a preselected metallic content in an induction furnace, melting the article by induction heating in a relatively short period of time, forming droplets of molten metal, subjecting the molten metal to a centrifugal force by a rotating disc, cooling the droplets into discrete superalloy metal powder particles, collecting the metal particles and providing an inert gas atmosphere having less than 1 part per million ofoxygen, water and nitrogen throughout the foregoing process.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a block diagram of typical equipment utilized in this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT For a better understanding of the present invention together with other and further objects, advantages and capabilities thereof, reference is made to the following specification and appended claims in connection with the aforedescribed drawing.

As was previously mentioned an inert atmosphere is provided throughout the process, although any atmosphere which contains less than about 1 part per million ofany reactive material or gas which would be included in the metal matrix can be used, the inert gases such as argon, neon and the like, or an extremely high vacuum that is about 5 X 10 torr or less is preferred. As is believed to be apparent it is important to maintain the equipment vapor tight so that there is no leakage of air into the processing equipment. By providing the inert gas atmosphere there can be no gaseous impurity pickup during the processing of the powder.

A shaped article of the desired metallic content, such as a bar of IN-lOO, a trademark of International Nickel Company, is placed into the center of the induction coil of an induction furnace. Although the article can be of any particular shape it is generally preferred to lower a bar through an induction coil. This invention has an advantage over melting the alloy in a crucible such as done in the prior art by achieving the melting in an inert atmosphere which eliminates contamination by impurities which can be present in a crucible. As the bar passes through the induction coil it is essentially instantaneously melted, thereby preventing volatilization of certain of the alloying elements which tend to volatilize during the prolonged heating required in many of the prior art processes. Since the powder formation is conducted in an inert atmosphere the raw material, the melt and the powder have the same chemical composition. None of the essential alloying elements are lost nor is there any pickup of undesirable contamination or impurities. In many of the superalloys slight deviations from the desired metal amounts and the presence of certain impurities can result in major changes in properties of the superalloys.

The essential portion of providing a centrifugal force to the molten material can be achieved by a variety of means. The generally preferred method is by providing a rotating disc which rotates at a relatively high speed. When the molten metal falls on the disc, the centrifugal force causes the melt to break up into fine droplets. It has been determined that the higher the speed of the rotation the greater amount of powder that is produced. Additionally, the fineness of the powder is increased with increased centrifugal force.

Although it is essential that the centrifugal force be provided to the molten material before it has had a chance to solidify to form a solid, it has generally been observed the greater the distance the melt has to travel to the centrifugal force the finer the droplets, therefore, the particle size of the powder produced is reduced. After the droplets are formed they are discharged from the rotating disc or any other type of centrifugal force that is used and fall through a cooler zone adjacent to the force. The droplets are solidified to discrete superalloy metal powder particles. One of the advantages of this process over the prior art processes is that even if undersize or oversize materials are produced these out-of-specification materials can be recycled, if desired, because of the essentially identical metallurgical composition as the original material. These particles, of course, have to be formed into a shape which can be induction melted to provide a closed loop process.

To more fully explain the subject invention, the following detailed examples are presented. All percentages, proportions and parts are by weight unless otherwise indicated.

EXAMPLE 1 The overall processing equipment is shown in FIG. 1. It consists of a powder making containment vessel with a transfer chamber 12. The transfer chamber connects the powder production vessel to a glove box 14. All of these individual components are attached to an inert gas purification system 16. This purification system will reduce oxygen, water and nitrogen in an inert gas to 1 ppm by volume or less. A typical inert gas system is manufactured by Vacuum Atmospheres Company, Hawthorne, California. With this interconnection of the powder production vessel and the glove box, it is possible to manufacture, transfer and handle the powder completely in an inert gas system which contains 1 ppm of less of oxygen, water and nitrogen. The purposeof this interconnection and high purity inert atmosphere is to prevent interstitial gas contamination of the powder during production or during subsequent secondary handling operations.

Another feature of this invention considered a key part in retaining low interstitial contents of the powder is the method of melting the material. In this invention the powder is produced from a metal bar of the proper composition. The bar is melted by lowering it through an induction coil. During the passage of the billet through the induction coil there is essentially instantaneous melting. Thus, the powder produced is of the composition as the billet. As the metal alloy is melted, it falls from the coil height and strikes a rotating disc. This disc breaks up the metal powder both from centrifugal force of the disc transferred to the molten alloy and from the impact of the molten alloy onto the disc. The powder flies from the disc and solidifies in flight prior to falling into the collection and/or transfer chambers.

Utilizing the equipment described, a bar of a low alloy superalloy is lowered through the induction coil, melted and powder produced. It has been found that as the rotational speed of the disc is increased the amount of powder produced is increased. The fineness of the powder is increased with increased disc speed. Also, as the height of the coil above the disc is increased, the amount and fineness of the powder produced is increased. Too fast a feed rate through the induction coil does not give good uniform melting and thus prevents a fine powder from being produced.

The following quantitative powder-making results are obtained and the conditions held constant are billet feed rate through coil at 0.875 inch per min., coil height above disc 3 inches, coil location at 6 inches on a 9 inch radius disc. The induction coil is a nine turn copper inch D coil. Runs are made at disc speeds of from 970 to 2000 RPM and the fraction in the 20 to mesh range is found to increase with an increase in disc speed.

To investigate the effect of billet feed rate, runs are made at 1.6 inches per minute and 0.25 inches per minute. The disc speed is held constant at 2000 RPM with the coil location identical to that cited earlier. The powder yields are not significantly different from those obtained at the 0.875 inches per minute. Thus, it can be said that the range of 0.25 to 1.6 inches per minute satisfactory melting is achieved and satisfactory powder is obtained.

EXAMPLE 2 Runs are conducted in the apparatus disclosed in Example l which yielded powder made from bar stock of a superalloy sold under the name of Astroloy by Cannon-Muskegon Co. The following table presents the particle size distribution obtained for powder produced in an argon atmosphere and a vacuum atmosphere at 5 X 10' torr pressure. The conditions are identical for both types of atmosphere: bar insertion speed into the induction coil: 0.875 in./min.; coil height above the disc about 4 /2 inches; drop point on a 9 in. radius disc is 6 inches from the center, and disc speed: about 2400RPM.

Table I Particle Size Distribution Table l-Continued Particle Size Distribution The prealloyed powder is the same composition as l the billet. No volatilization of essential alloying elements occurs.

Element Alt-Received Billet Other runs are made with tll e ctiil 8 tb l0 inchesabove the disc. Disc speed is held constant at 3400 RPM to allow comparison with previous results. The coil design is identical to the previous runs.

With the coil 8 to 10 inches above the disc, the 4 run 'average of 20+60 mesh powder yield is 62.9% in vacuum compared to 60.5 for one run at acoil height of 4-5 inches. However the total below l00 mesh averages 20.2% at 8 to [0 inch coil height vs. 16.8% at 4 to inch coil height. For the one argon experiment. 64.5% of the powder is -20+60 mesh at 8 to inches vs. 67% at 4 to 5 inches. There is a 1.5% increase in l00 mesh powder with greater coil height.

During the experiments with the 4 to 5 inch coil, the

large, unmelted pieces of billet are removed prior to Ni base i base weighing the powder and the percentages below go :28: weight 17! mesh is deterred. The practice with the 8 to 10 inch coil Ti 3,7 3, 3:75 height experiments is maintained. Also the unmelted Q g -g 5 3 tchunks are weighed so that an overall efficiency of bilo 20 let to 20 mesh powder could be calculated. For the four vacuum runs, 39% of the material melted is converted to 2 mesh powder. This figure is lowered to EXAMPLE 3 21% if the unmelted billet pieces are included in the All the data and discussion in this section is based on +20 me h scrap ot l- F or he single arg n r n th figpowder production runs using IN-100 ba t k, mures are 43.5% and 20.5%. Thus 1t ls shown that the nally 2% inch D, which is grit blasted to remove surface Coil height does not Change the yield Of "-20 mesh powoxide prior to converting it to powder. der for either argon or vacuum atmospheres. However, The first step is to quantitatively evaluate disc speed the greater coil height does increase the yield of l 00 and argon vs. vacuum as they influenced powder yield mesh powder 1n both atmospheres. In all cases, about and particle size distribution, Runs 3, 4, and 5 in argon of the 200 mesh powder ls 325 mesh. and runs 6 through 9 in vacuum are conducted at disc Runs 18 and 19 were completed with Astroloy rather speeds of 2500, 3000, and 3400 RPM. For both argon than lN-lOO. and vacuum atmospheres the incgease disc peed The data are summarized 1n Table II following:

TABLE II Disc Coil Powder Size Dist. Run Speed Height 20 Mesh Powder Mesh Size No. (RPM) Am. (in.) Weight (grams) -20+60 +100 -|OO+200 20() 3 2500 Argon 4-5 738.0 78.5 16.2 4.3 0 8 4 3000 Argon 4-5 600 76.5 15.3 6.8 1 5 5 3400 Argon 4-5 1026 67.0 20.0 9.2 4 0 6 2500 VflC- 4-5 1344 64.5 16.5 10.8 8 2 uum 7 3000 Vac- 4-5 840 61.0 33.0 3.8 2 2 uum 8 3000 Vac- 4-5 1447 63.6 17.6 10.6 8.1

uum 9 3400 Vac- 4-5 1861 60.5 22.7 12.0 4.8

uum 12 3400 Vac- 8-l0 2036 64.8 19.5 5.0 10.8

uum 13 3400 VflC- 8-10 l360 62.5 16.5 8.4 12.6

uum 14 3400 Argon 8-10 920 64.5 20.9 9.6 5.0 15 3400 Vac- 8-10 885 61.4 17.4 9.4 12.0

um 16 3400 Vac- 8-10 663 63.0 14.3 11.0 11.6

uum 18 3500 Argon 8-10 1089 73.5 19.0 6.2 1.3 19 3500 Vac- 8-10 455 74.3 18.9 6.1 0.7

uum

from 2500 to 3400 RPM produces a greater yield of -60 mesh particles. It is also apparent that the vacuum atmosphere is conductive to a slight increase, relative to argon, in the -60 mesh fractions. For both atmospheres 4071 of the melted alloy, excluding large pieces of the billet, is -20 mesh powder. These runs are made .with the coil 4 to 5 inches above the disc and the billet spe d s set 1. 15. c s P m nu e st! rats-t...

more susceptible to shear forces from the disc and from impact which breaks the melt into finer powder particles. It is also noted that the particles in vacuum are much hotter as they are collected in the chamber than those produced in argon.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

l. A process suitable for producing prealloyed superalloy powders, said process comprising conducting in an inert atmosphere containing less-than about 1 part per million of oxygen,-water and nitrogen the following processing steps:

a. placing a shaped article of a superalloy having a preselected metallurgical content in an induction furnace,

b. induction melting a portion of said article at a consphere is a vacuum atmosphere at least about 5 X 10' Torr pressure.

4. A process according to claim 3 wherein said article is a bar of superalloy and is melted at the rate of between about 0.25 to about 1.6 inches per minute.

5. A process according to claim 4 wherein said particles are produced by dropping the molten metal on a disc rotating at from about 1500 to about 4000 revolutions per minute. 

1. A PROCESS SUITABLE FOR PRODUCING PREALLOYED SUPERALLOY POWDERS, SAID PROCESS COMPRISING CONDUCTING IN AN INERT ATMOSPHERE CONTAINING LESS THAN ABOUT 1 PART PER MILLION OF OXYGEN, WATER AND NITROGEN THE FOLLOWING PROCESSING STEPS, A. PLACING A SHAPED ARTICLE OF A SUPERALLOY HAVING A PRESELECTED METALLURGICAL CONTENT IN AN INDUCTION FURNACE, B. INDUCTION MELTING A PORTION OF SAID ARTICLE AT CONTROLLED RATE BY INDUCTION HEATING, C. FORMING DROPLETS OF MOLTEN METAL BY SUBJECTING THE MOLTEN METAL TO A CENTRIFUGAL FORCE BY A ROTATING DISC, D. COOLING SAID DROPLETS TO FORM SOLID PARTICLES OF SUPERALLOY, AND E. COLLETING SAID PARTICLES.
 2. A process according to claim 1 wherein said inert atmosphere is argon.
 3. A process according to claim 1 wherein said atmosphere is a vacuum atmosphere at least about 5 X 10 4 Torr pressure.
 4. A process according to claim 3 wherein said article is a bar of superalloy and is melted at the rate of between about 0.25 to about 1.6 inches per minute.
 5. A process according to claim 4 wherein said particles are produced by dropping the molten metal on a disc rotating at from about 1500 to about 4000 revolutions per minute. 